This invention relates to an optical resin material excellent in transparency, moldability, heat resistance and moisture resistance and suitable for optical element materials with low birefringence and high reciprocal dispersion.
As transparent thermoplastic resins, particularly resins for optical elements, there have been known poly(methyl methacrylate), polystyrene, polycarbonate and the like. But transparent resins satisfying all the required properties, that is, low birefringence, low moisture absorption, good molding and processing properties as well as transparency and heat resistance, have not been obtained yet.
For example, poly(methyl methacrylate) is excellent in transparency and low birefringence, and high in reciprocal dispersion, and thus a relatively excellent material. However, it has a problem in that a dimensional change readily takes place during use due to large moisture absorbing properties. Polystyrene resins are very small in moisture absorption but large in birefringence and easily bring about a change in transparency with the lapse of time. Therefore, such resins are not suitable as materials for optical elements. Polycarbonate resins are very excellent in heat resistance and excellent in moisture resistance but large in birefringence and slightly inferior in transparency. Further, since polystyrene and polycarbonate are low in reciprocal dispersion, when these resins are used as optical lenses, it is difficult to use them as master lenses which control the performance of optical system.
Among these resins, poly(methyl methacrylate) resins have high possibility of being used for optical uses. In order to improve the moisture resistance of poly(methyl methacrylate) resins, various proposals have been disclosed, for example, in Japanese Patent Unexamined Publication Nos. 58-5318, 58-5354, 58-11515, 58-13652, and 59-122509. One example of these proposals is a process for copolymerizing cyclohexyl methacrylate, benzyl methacrylate or a higher alkyl ester of methacrylic acid with methyl methacrylate. According to these proposals, the moisture resistance of the poly(methyl methacrylate) resins may be improved considerably but is still insufficient as optical element materials, while the heat resistance is remarkably lowered.
On the other hand, a polymer containing tricyclo[5.2.1.0.sup.2,6 ]deca-8-yl acrylate or methacrylate as an essential component is disclosed in U.S. Pat. No. 4,591,626 as an optical resinous material excellent in moisture resistance and heat resistance. However, said polymer is still insufficient for providing high impact molded articles.